Wide-gap semiconductor materials including silicon carbide (SiC) have such excellent characteristics as their high dielectric breakdown field strengths, which are about ten times higher than that of silicon (Si), and thus have been drawing attention as materials suitable for high withstand voltage bipolar power semiconductor devices having high withstand reverse voltage characteristics.
Bipolar semiconductor devices such as pin (Positive Intrinsic Negative) diodes, bipolar transistors, GTO (Gate Turn-Off Thyristor) and GCT (Gate-Commutated Thyristor), although high in built-in voltage as compared with unipolar semiconductor devices such as Schottky diode and MOSFET, yet have a characteristic that their on-resistance is largely decreased by conductivity modulation of the drift layer due to minority carrier injection. Accordingly, in high-voltage, large-current regions for power use or the like, bipolar semiconductor devices are used with a view to reducing losses. These bipolar semiconductor devices, when made from SiC, are enabled to implement far more excellent performance, compared with devices of Si. For example, a pin diode having a high withstand voltage of 10 kV and made from SiC has a forward voltage amounting to about one third that of Si pin diodes. The SiC pin diode shows a high-speed inverse recovery time, which represents operating speed at a turn-off, amounting to about 1/20 or less of that of the Si pin diode. Also, the SiC pin diode shows a power loss amounting to about ⅕ or less of that of the Si pin diode, greatly contributing to energy saving. In addition to SiC pin diodes, SiC npn transistors, SiC SIAFETs, SiC SIJFETs and the like have been developed as well, and similar power loss reduction effects have been reported (e.g., non-patent document 1). Otherwise, SiC GTOs using a p-type semiconductor layer of inverse polarity as a drift layer have also been developed (e.g., non-patent document 2).
A {0001} surface, which represents a set plane of SiC crystal, includes a (0001) silicon surface 1 and a (000-1) carbon surface 2, which are individual surfaces, as shown in a perspective view of SiC crystal of FIG. 7. In this case, a parenthesized symbol ‘−’ represents a negative sign, which is referred to as polarity. The (0001) silicon surface 1 is a surface in which the crystal is terminated by silicon (Si). The (000-1) carbon surface 2 is a surface in which the crystal is terminated by carbon (C). Nitrogen (N), which is an N-type dopant, is incorporated into the SiC crystal to substitute carbon (C) in most cases. The (0001) silicon surface 1 terminated by silicon is smaller in the quantity of surface-appearing carbon than the (000-1) carbon surface 2 terminated by carbon. Therefore, substitution of carbon into nitrogen (N) is suppressed so that a high-purity epitaxial layer can be obtained. On account of this, most of study reports relating to epitaxial growth of SiC are associated with the (0001) silicon surface 1.
In SiC epitaxial growth, the CVD (Chemical Vapor Deposition) process, which allows easier control of the growth rate and the purity of the epitaxial layer, is used. However, since the CVD process employs hydrogen as the carrier gas, separation of carbon (C) from the growth surface occurs during the growth, causing the growth rate to be suppressed. Thus, the normal growth rate is 5 to 10 μm/h.
For fabrication of the conventional bipolar semiconductor device with SiC, for example, n-type 4H-SiC which has been formed so as to have a surface la whose off-angle θ from the (0001) silicon surface 1 is 8 degrees is used for the substrate. In the term ‘4H’, numeral ‘4’ represents a crystal structure in which atomic layers are of a four-layer cycle, and ‘H’ refers to hexagonal crystal. On this substrate, a SiC drift layer for relaxation of electric field in voltage application is formed by epitaxial growth at a growth rate of 5 to 10 μm/h by using CVD process.
Non-patent document 1: Hiroyuki MATSUNAMI (editor and writer), “Semiconductor SiC technology and applications,” edited and written by pp. 218-221, Nikkan Kogyo Shimbunsha.
Non-patent document 2: “A. K. Agarwal et. al, Materials Science Forum Volume 389-393, 2002, pp. 1349-1352.